The scale of the Islamic State’s operations in Europe are still not known, but they appear to be larger and more layered than investigators at first realized; if the Paris and Brussels attacks are any model, the plotters will rely on local criminal networks in addition to committed extremists.

But it also was a reminder of the ease with which the Islamic State’s operatives move across borders and the shifting roles that suspects play: According to prosecutors, Mr. Abrini was a logistician in the Paris attacks but was meant to be a bomber in the Brussels attack — except that his bomb failed to explode.

“Other Islamic State cells are highly likely to be in existence across Western Europe, preparing and organizing further operations, and awaiting direction from the group’s central leadership to execute,” said Matthew Henman, the head of IHS Jane’s Terrorism and Insurgency Center in London.

It adds up to a long road ahead in Europe for law enforcement and intelligence agencies but also for citizens who are having to learn to adapt to an array of new security precautions and more intrusive surveillance, especially in public places.

“We are not finished yet with the job of finding everyone who is in this big network of Paris and Brussels,” said Jean-Charles Brisard, the head of the French Center for the Analysis of Terrorism in Paris. “Every time progress is made, we add another few people to the list of people we are looking for.”

“There are still many people involved who were part of the Zerkani network, who were convicted in absentia — at least five to 10 — and we don’t know where they are or what they might do,” Mr. Brisard said.

Kriket had Kalashnikov assault rifles, a submachine gun, pistols, ammunition and four boxes containing thousands of small steel balls.

Four men in touch with Mr. Kriket, who were arrested in Rotterdam in the Netherlands, had in their possession 45 kilograms of ammunition, according to the Dutch Public Broadcaster, NOS. That is enough ammunition for 2,500 rounds, which is enough to supply as many as a dozen gunmen with multiple magazines.

“Other Islamic State cells are highly likely to be in existence across Western Europe, preparing and organizing further operations, and awaiting direction from the group’s central leadership to execute,” said Matthew Henman, the head of IHS Jane’s Terrorism and Insurgency Center in London.

Officials believed that the Islamic State had developed an overarching network of facilitators in Europe over the last few years to buy weapons, rent cars and reserve hotel rooms for teams of operatives who had previously traveled to, or were returning from, Iraq or Syria.